Beer, carbonated and non-carbonated soft drinks, and fruit juices (hereinafter referred to generically as beverages) are often packed in containers made from aluminum, tin-free steel, blackplate or tinplate, which is cold rolled steel to which a thin layer of tin is applied. Many of these beverages exert corrosive action upon the metal and in order to adequately protect the container and to prevent contamination of the packaged material, a sanitary liner must be applied to the internal surface of the container. However, the use of such liners also presents several problems, one of the most troublesome being the residual turbidity and taste which tends to result from some liner materials.
Because of their relatively taste-free characteristics and other excellent properties, epoxy resins have been extensively employed in sanitary liners in contact with beverages. While such epoxy resins have been extremely useful in the past, they possess a serious disadvantage which materially diminishes their desirability as sanitary liners at this time. Thus, these epoxy resins are generally applied from volatile organic solvent solutions at relatively low solids contents and these solvent rich solutions either add to hydrocarbon air pollution or require expensive control equipment.
In recent times the increased emphasis on safety and environmental pollution control has resulted in a need for water-based compositions for such liners. By "water-based" it is meant compositions in solvents consisting predominantly of water, thus greatly reducing the handling and emissions of organic solvent vapors. However, the types of solvent-based epoxy resin liners known and used heretofore have not been readily obtainable as satisfactory water-based systems and, indeed, it has been found that water-based materials as a class often provide liners which impart undesirable turbidity and taste characteristics to beverages, even when the other necessary properties of such liners can be obtained.
The combination of properties which is necessary to successful utilization of any composition for container liners is as follows:
A. PROPERTIES OF THE CURED LINER:
1. Metal Adhesion--Excellent adhesion to metals, including the aluminum, tin-free steel, blackplate and tin plate employed in beverage containers.
2. Taste Characteristics--Taste characteristics at least as good as the best "tasteless" epoxy polymers applied from solvent solutions utilized--in the container industry at the present time.
3. Turbidity Resistance--Beverages after packing, pasteurization and storage must not develop undesirable turbidity and loss of appearance due to contact with the liner.
4. Fabricating Properties--Fabricating properties represent a combination of flexibility, extensibility and adhesion so as to permit forming operations to be carried out on the coated metal without cracking or otherwise impairing the continuity of the film.
5. Pasteurization Resistance--Beer is generally pasteurized at a temperature of 150.degree. F. for 15 to 40 minutes; occasionally during the pasteurization temperatures as high as 160.degree. F. to 180.degree. F. may be reached.
6. Low Bake Properties--The curing or baking temperature in metal beverage containers should not be excessively high because the exterior of some containers may be coated with lithographic coatings and inks which may discolor and lose their appearance at high temperatures. In addition, some containers employ adhesives as bonding agents and such adhesives may be adversely affected by high baking temperatures.
7. Extractability--The liner should not contain undesirable materials which can be extracted from the liner during processing and storage.
8. Intercoat Adhesion--In order to permit use of primer or base coat, if desired, or added coats to repair defects, the liner composition should have good adhesion to itself and other conventionally utilized materials.
B. PROPERTIES OF THE UNCURED COMPOSITION:
1. Application Properties--Application by equipment and methods conventionally employed in the coatings industry. Thus, the composition should be capable of being applied by methods such as dipping, roll coating, spraying and the like. In addition, the composition should be capable of being applied by electrodeposition if desired.
2. Storage Stability--The coating composition must be in a physical form which permits handling and storage over varying conditions. Water-based compositions in emulsion form, for example, usually are not storage-stable unless additives are employed which generally are undesirable in liners for containers used for comestible products.